To respond to increasing communication demands, for example, a communication service enabling large capacity data transmission between routers by coupling the routers via the wavelength division multiplexing (WDM) network is used widely. The WDM network includes a reconfigurable optical add and drop multiplexer (ROADM) that wavelength multiplexes and transmits a plurality of optical signals having different wavelengths and a transponder that transmits and receives an optical signal for each wavelength to and from the ROADM.
The transponder is, for example, coupled with a router in the client network. Upon detecting non-setting or fault of the optical line (or optical path) between ROADMs, the transponder terminates the link with the router by a function of the link pass through. This allows the router to detect non-setting or fault of the optical line and thus promptly change the path of packets in accordance with a dynamic routing protocol such as, for example, an Open Shortest Path First (OSPF).
When providing a communication service (such as wide area Ethernet service) of Ethernet (registered trademark: the same applies hereinafter) between routers, the adjacency relationship between the transponder and the router has to be properly recognized prior to setting of the optical line to identify the coupling target by the optical line. Related techniques are disclosed in, for example, Japanese Laid-open Patent Publication No. 2014-27471.